Once considered old-fashioned and ungainly, rose of Sharon (Hibiscus syriacus, USDA zones 5-9) has regained its popularity in large part due to recent introductions from the National Arboretum. Cultivars such as 'Diana' and 'Aphrodite,' with their larger flowers and more refined habit, have encouraged a new generation of gardeners to grow this dependable flowering shrub. Lightly fertilizing plants twice a year is really the only attention rose of Sharon needs to grow well in most gardens.
To optimize growth and flowering, fertilize your rose of Sharon in the spring once the plant emerges from dormancy and starts to sprout leaves. Another fertilizer application in midsummer helps encourage continued blooming.
Rose of Sharon Culture
Rose of Sharon is a tough plant. It thrives in full sun or light shade, on moist, well-draining soil high in organic matter; but it's so adaptable it grows well even in dry, compacted urban gardens. Rose of Sharon is rarely bothered by pests or diseases, although old shrubs may develop stem cankers, a sign that the plant is declining and should be replaced with a new specimen.
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A heat lover, rose of Sharon is late to leaf out in the spring. It blooms on the current season's wood, so you can prune it heavily in early spring to control size or shape.
Choosing a Fertilizer
Rose of Sharon benefits from two different fertilizer formulations. For its spring boost, use a light application of a balanced fertilizer such as 10-10-10, if soil-test results recommend this. In midsummer give it a boost with a low-nitrogen fertilizer such as 0-10-10. Rose of Sharon is susceptible to bud drop and aphid infestations if over-fed, so be conservative with fertilizer amounts. Using a slow-release fertilizer formula can cut down on the risk of over-fertilizing your rose of Sharon and helps supply nutrients consistently.
When to Fertilize
As with any deciduous shrub, rose of Sharon's initial spring feeding shoulld be done after the plant has broken dormancy and the leaves are emerging, as this is the most effective time to fertilize. If you fertilize the plant in late fall or earlier in the spring, those feedings are less effective, as the plant is not in active growth. Some of the fertilizer nutrients will leach away before the plant absorbs them. Fertilizing the shrub a second time in midsummer with a low-nitrogen fertilizer encourages continued bloom production.
Rose of Sharon Fertilizer Precautions
To prevent chemical burns to roots, the soil around any plant should be moist before you apply the fertilizer. If the weather has been dry, water the shrub well the day before you plan to fertilize. After applying a granular fertilizer, water again to move the fertilizer down into the soil. Always follow the package directions carefully for the correct amount of product to use. With fertilizer, more is not better.
If you're getting lots of leaf growth but few flowers, the shrub is either not getting enough sunlight or it's getting too much nitrogen. Make sure that lawn fertilizer is not being applied over the shrub's root zone.